This invention relates generally to automatic gain control (AGC) circuits and more specifically to the control of feedforward AGC circuits used with SSB receivers. This invention is particularly suited for use with receivers which receive amplitude modulated (AM) signals including single sideband (SSB) signals.
The basic purpose of an AGC circuit in a SSB receiver is to minimize undesired audio output variations caused by changes in signal strength. Two basic types of AGC circuits are used: feedback and feedforward circuits. In a feedback AGC circuit a control signal is used to regulate the gain of a receiver stage which precedes the stage where the control signal is derived. A feedforward AGC circuit is similar to the feedback AGC circuit except that its control signal regulates the gain of a stage which follows the stage where the control signal was derived. For relatively slow signal strength fluctuations, feedback AGC is effective. Feedforward AGC is utilized when rapid signal strength variations, such as caused by multipath interference (Rayleigh fading), must be neutralized.
The use of feedback and feedforward ACG circuits in SSB receivers is known; for example, see the article "An SSB With Pilot Recievers For Mobile Radio" by J. R. Ball and D. W. J. Holmes in July, 1981 IERE Conference on Radio Receivers and Associated Systems, pages 429-435, and U.S. Pat. No. 4,313,211 to K. W. Leland.
Although feedforward AGC compensates effectively for signal fades at moderate and strong signal levels, excessive noise (noise bursts) occurs in the receiver audio output during weak signal level conditions. In a SSB system using a pilot signal, the presence of noise destroys the correlation that had existed between the amplitude fluctuations of the carrier or pilot signal and the fade induced amplitude fluctuations of the voice signal. Noise bursts occur when a noise peak exists in the voice band at the same time that a minimum exists in the noise present in the pilot band.